The present invention relates to the epoxidation of an olefin such as propylene by reaction with hydrogen and oxygen using a solid catalyst such as Pd on TS-1, the improvement being that the reaction is carried out in carbon dioxide solvent under dense reaction mixture phase conditions.
Epoxides constitute an important class of chemical intermediates useful for the preparation of polyether polyols, glycols, glycol ethers, surfactants, functional fluids, fuel additives and the like. Many different methods for synthesizing epoxides from the corresponding olefins have been described in the literature. A Japanese patent application assigned to the Tosoh Corporation and published in 1992 (Kokai No. 4-352771) proposed making propylene oxide by reacting propylene, hydrogen and oxygen using a catalyst comprising a Group VIII metal and a crystalline titanosilicate.
As with any chemical process, it would be desirable to attain further improvements in epoxidation methods of this type.
Dense phase reaction mixture conditions have been employed in various reaction systems, most notably in the production of tertiary butyl hydroperoxide by direct oxidation of isobutene. See, for example, U.S. Pat. Nos. 4,408,081 and 4,408,082.
In accordance with the present invention, the epoxidation is carried out by reacting olefin, hydrogen and oxygen using a noble metal on titanium or vanadium zeolite catalyst, the improvement being that the reaction is carried out using CO2 as the essential solvent at dense phase reaction conditions.
There are a number of significant advantages which are achieved through practice of the present invention. CO2 is the essential solvent used for the reaction and accordingly solvolysis of the oxirane product is suppressed due to the absence of any nucleophile species except for water of reaction. Leaching of noble metal from the solid catalyst is minimal due to insolubility in CO2. Because the olefin, hydrogen and oxygen are totally miscible in the dense phase system, better control of the reagent concentrations can be achieved and the dead space in the reactor can be substantially eliminated.
In general, reagents and catalysts previously taught for this reaction can be used. In this regard, reference is made to prior teachings such as Kokai No. 4-352771 above referred to as well as U.S. Pat. Nos. 6,005,123 and 6,008,388, the disclosure""s of which are incorporated herein by reference.
The catalysts to be used in the present process are comprised of a titanium or vanadium zeolite and a noble metal (preferably an element of Group VIII of the Periodic Table). Suitable zeolites are those crystalline materials having a porous molecular sieve structure with titanium or vanadium atoms substituted in the framework. The choice of zeolite employed will depend upon a number of factors, including the size and shape of the olefin to be epoxidized. For example, it is preferred to use a relatively small pore titanium or vanadium zeolite such as a titanium silicalite if the olefin is a lower aliphatic olefin such as ethylene, propylene, or 1-butene. Where the olefin is propylene, the use of a TS-1 titanium silicalite or vanadium silicalite is especially advantageous. For a bulky olefin such as cyclohexene, a larger pore titanium zeolite such as a titanium zeolite having a structure isomorphous with zeolite beta may be preferred.
The titanium-containing zeolites useful as catalysts in the epoxidation step of the process comprise the class of zeolitic substances wherein titanium atoms are substituted for a portion of the silicon atoms in the lattice framework of a molecular sieve. Such substances are well known in the art.
Particularly preferred titanium-containing zeolites include the class of molecular sieves commonly referred to as titanium silicalites, particularly xe2x80x9cTS-1xe2x80x9d (having an MFI topology analogous to that of the ZSM-5 aluminosilicate zeolites), xe2x80x9cTS-2xe2x80x9d (having an MEL topology analogous to that of the ZSM-11 aluminosilicate zeolites), and xe2x80x9cTS-3xe2x80x9d (as described in Belgian Pat. No. 1,001,038). Also suitable for use are the titanium-containing molecular sieves having framework structures isomorphous to zeolite beta, mordenite, ZSM-48, ZSM-12, and MCM41. The titanium-containing zeolite preferably contains no elements other than titanium, silicon and oxygen in the lattice framework, although minor amounts of boron, iron, aluminum, and the like may be present. Other metals such as tin or vanadium may also be present in the lattice framework of the zeolite in addition to the titanium, as described in U.S. Pat. Nos. 5,780,654 and 5,744,619.
Preferred titanium-containing zeolite catalysts suitable for use in the process of this invention will generally have a composition corresponding to the following empirical formula xTiO2 (1xe2x88x92x)Sio2 where x is between 0.0001 and 0.500. More preferably, the value of x is from 0.01 to 0.125. The molar ratio of Si:Ti in the lattice framework of the zeolite is advantageously from 9.5:1 to 99:1 (most preferably from 9.5:1 to 60:1). The use of relatively titanium-rich zeolites may also be desirable.
While any of the noble metals can be utilized (i.e., gold, silver, platinum, palladium, iridium, ruthenium, osmium), either alone or in combination, palladium is particularly desirable. Typically, the amount of noble metal present in the catalyst will be in the range of from 0.01 to 20 weight percent, preferably 0.1 to 5 weight percent. The manner in which the noble metal is incorporated into the catalyst is not considered to be particularly critical. For example, the noble metal may be supported on the zeolite by impregnation means or the like or first supported on another substance such as silica, alumina, activated carbon or the like and then physically mixed with the zeolite. Alternatively, the noble metal can be incorporated into the zeolite by ion-exchange with, for example, Pd tetraamine chloride with or without added ammonium hydroxide. The catalyst is recovered by filtration and washing and is substantially free ( less than 0.1 wt. %) of halide. There are no particular restrictions regarding the choice of noble metal compound or complex used as the source of the noble metal. For example, suitable compounds for such purpose include the nitrates, sulfates, halides (e.g., chlorides, bromides), carboxylates (e.g., acetate), and amine complexes of noble metals. Similarly, the oxidation state of the noble metal is not considered critical. In the case of palladium for instance, the palladium may be in an oxidation state anywhere from 0 to +4 or any combination of such oxidation states. To achieve the desired oxidation state or combination of oxidation states, the noble metal compound after being introduced into the catalyst may be fully or partially pre-reduced. Satisfactory catalytic performance can, however, be attained without any pre-reduction whatsoever. To achieve the active state of palladium, the catalyst may undergo pretreatment such as thermal treatment in nitrogen, vacuum, hydrogen or air.
The catalyst may additionally comprise a binder or the like and may be molded, stray dried, shaped or extruded into any desired form prior to use in epoxidation. In addition to the noble metal, the catalyst may be modified with additional elements such as, for example, lanthanide metals (e.g., europium) iron, cobalt, nickel, boron, aluminum, phosphorus, calcium, vanadium, chromium, manganese, copper, zinc, and gallium.
The olefin to be used can be any organic compound containing at least one site of ethylenic unsaturation (i.e., at least one carbon-carbon double bond). The olefin can be aliphatic, aromatic or cycloaliphatic in character and may have either a linear or branched structure, with the site(s) of theylenic unsaturation being terminal and/or internal. The olefin preferably contains 2-30 carbon atoms; the process of the invention is particularly suitable for expoxidizing C2-C6 mono-olefins. More than one double bond may be present, as in a diene or triene for example. The olefin may be a hydrocarbon (i.e., contain only carbon and hydrogen atoms) or may contain functional groups such as halide, carboxyl, hydroxyl, ether, carbonyl, cyano, or nitro, groups or the like.
Typical examples of suitable olefins include ethylene, propylene, 1-butene, cis- and trans-2-butene, isobutene, 1,3-butadiene, pentenes, isoprene, hexenes, octanes, nonenes, decenes, undecenes, dodecenes, cyclopentene, cyclohexene, dicyclopentadiene, vinylcylohexane, vinylcyclohexene, allyl chloride, allyl alcohol, methallyl chloride, methallyl alcohol, alkyl acrylates and methacrylates, unsaturated fatty acids and esters thereof, styrene, alpha methylstyrene, divinylbenzene, indene and stilbene. Mixtures of olefins may, of course, be utilized if so desired. The process of this invention is especially useful for converting propylene to propylene oxide.
The epoxidation reaction is carried out using CO2 as essential solvent, at conditions which are effective to provide a single dense phase reaction mixture. Although small amounts of other solvents can be tolerated it is preferred that CO2 comprise the major amount and preferably at least 98% by weight of the reaction solvent. The CO2 solvent comprises at least about 10 wt % of the reaction mixture, preferably about 75% or more up to 95% or more.
A dense-phase feed reaction mixture is one maintained at a pressure sufficiently elevated so that the mixture of olefin xe2x80x94CO2xe2x80x94O2xe2x80x94H2 is a single, dense, liquid-like phase with a density higher than 0.25 g/cm3. Suitable temperatures are in the range of 20 to 100xc2x0 C. and suitable pressures from 10 to 300 atm. In other words, the dense-phase reaction mixture is free of the vapor phase, which has been characteristic of prior art reaction in the vapor phase or in a two phase (vapor-liquid) mixture. Preferably, the feed reaction mixture is under supercritical conditions, that is, a mixture comprising of olefin, oxygen, hydrogen, CO2 maintained above its critical temperature as well as above its critical pressure. The oxygen and hydrogen are present in solution in the dense phase mixture, not as a separate vapor phase.
Although the critical points of temperature and pressure of CO2 of 31.1xc2x0 C. and 72.9 atm are useful in establishing conditions for carrying out the reaction in accordance with the invention, actually the critical temperature of the feed solution, for example propylene in CO2, is between 31.1xc2x0 C. and 91.9xc2x0 C. (Tc of propylene) and the critical pressure is higher than 46.2 atm (Pc of propylene). In the case of propylene epoxidation, suitable operating conditions are pressure of 15 to 300 atm and temperatures of 20 to 100xc2x0 C. The molar ratio of hydrogen to oxygen can usually be varied in the range of H2:O2=1:10 to 5:1 and is especially favorable at 1:5 to 2:1. The molar ratio of oxygen to olefin is usually 1:1 to 1:20, and preferably 1:1.5 to 1:10.
The following examples illustrate practice of the invention.